From 1971 to 1997, MEDLINE online access to the MEDLARS Online computerized database had been primarily through institutional facilities, such as university libraries. PubMed, first released in January 1996, ushered in the era of private, free, home- and office-based MEDLINE searching.[1] The PubMed system was offered free to the public in June 1997, when MEDLINE searches via the Web were demonstrated, in a ceremony, by Vice President Al Gore.[2]

Information about the journals indexed in MEDLINE and available through PubMed is found in the NLM Catalog.[6]

As of 8 February 2015[update], PubMed has over 24.6 million records going back to 1966, selectively to the year 1865, and very selectively to 1809; about 500,000 new records are added each year. As of the same date[update], 13.1 million of PubMed's records are listed with their abstracts, and 14.2 million articles have links to full-text (of which 3.8 million articles are available full-text for free for any user).[citation needed]

In 2016, NLM changed the indexing system so that publishers will be able to directly correct typos and errors in PubMed indexed articles.[7]

For comprehensive, optimal searches in PubMed, it is necessary to have a thorough understanding of its core component, MEDLINE, and especially of the MeSH (Medical Subject Headings) controlled vocabulary used to index MEDLINE articles. They may also require complex search strategies, use of field names (tags), proper use of limits and other features, and are best carried out by PubMed search specialists or librarians,[10] who are able to select the right type of search and carefully adjust it for precision and recall.[11]

Publication type parameter enables many special features. A special feature of PubMed is its "Clinical Queries" section, where "Clinical Categories", "Systematic Reviews", and "Medical Genetics" subjects can be searched, with study-type 'filters' automatically applied to identify substantial, robust studies.[12] As these 'clinical girish' can generate small sets of robust studies with considerable precision, it has been suggested that this PubMed section can be used as a 'point-of-care' resource.[13]

Since July 2005, the MEDLINE article indexing process extracts important identifiers from the article abstract and puts those in a field called Secondary Identifier (SI). The secondary identifier field is to store accession numbers to various databases of molecular sequence data, gene expression or chemical compounds and clinical trial IDs. For clinical trials, PubMed extracts trial IDs for the two largest trial registries: ClinicalTrials.gov (NCT identifier) and the International Standard Randomized Controlled Trial Number Register (IRCTN identifier).[14]

A reference which is judged particularly relevant can be marked and "related articles" can be identified. If relevant, several studies can be selected and related articles to all of them can be generated (on PubMed or any of the other NCBI Entrez databases) using the 'Find related data' option. The related articles are then listed in order of "relatedness". To create these lists of related articles, PubMed compares words from the title and abstract of each citation, as well as the MeSH headings assigned, using a powerful word-weighted algorithm.[15] The 'related articles' function has been judged to be so precise that some researchers suggest it can be used instead of a full search.[16]

A strong feature of PubMed is its ability to automatically link to MeSH terms and subheadings. Examples would be: "bad breath" links to (and includes in the search) "halitosis", "heart attack" to "myocardial infarction", "breast cancer" to "breast neoplasms". Where appropriate, these MeSH terms are automatically "expanded", that is, include more specific terms. Terms like "nursing" are automatically linked to "Nursing [MeSH]" or "Nursing [Subheading]". This important feature makes PubMed searches automatically more sensitive and avoids false-negative (missed) hits by compensating for the diversity of medical terminology.

LinkOut, a NLM facility to link (and make available full-text) local journal holdings.[19] Some 3,200 sites (mainly academic institutions) participate in this NLM facility (as of March 2010[update]), from Aalborg University in Denmark to ZymoGenetics in Seattle.[20] Users at these institutions see their institutions logo within the PubMed search result (if the journal is held at that institution) and can access the full-text.

PubMed/MEDLINE can be accessed via handheld devices, using for instance the "PICO" option (for focused clinical questions) created by the NLM.[22] A "PubMed Mobile" option, providing access to a mobile friendly, simplified PubMed version, is also available.[23]

For help using PubMed identifiers within Wikipedia, see Wikipedia:PMID.

A PMID (PubMed identifier or PubMed unique identifier)[25] is a unique integer value, starting at 1, assigned to each PubMed record. A PMID is not the same as a PMCID which is the identifier for all works published in the free-to-access PubMed Central.[26]

The assignment of a PMID or PMCID to a publication tells the reader nothing about the type or quality of the content. PMIDs are assigned to letters to the editor, editorial opinions, op-ed columns, and any other piece that the editor chooses to include in the journal, as well as peer-reviewed papers. The existence of the identification number is also not proof that the papers have not been retracted for fraud, incompetence, or misconduct. The announcement about any corrections to original papers may be assigned a PMID.

The National Library of Medicine leases the MEDLINE information to a number of private vendors such as Ovid, Dialog, EBSCO, Knowledge Finder and many other commercial, non-commercial, and academic providers.[27] As of October 2008[update], more than 500 licenses had been issued, more than 200 of them to providers outside the United States. As licenses to use MEDLINE data are available for free, the NLM in effect provides a free testing ground for a wide range[28] of alternative interfaces and 3rd party additions to PubMed, one of a very few large, professionally curated databases which offers this option.

Lu[28] identifies a sample of 28 current and free Web-based PubMed versions, requiring no installation or registration, which are grouped into four categories:

GoPubMed is a knowledge-based (Gene Ontology and MeSH) search engine for PubMed. GoPubMed claims to be a semantic search engine, but searches return exactly the same results as PubMed itself.

Expertscape provides search and ranking of medical and biomedical expertise by specific diagnosis, technique, or other terminology. Results are based on analysis derived from most recent ten years of PubMed data.[41]

Search term forwarders like "OssiPubMed online". O. Groth., which runs searches on multiple external platforms derived from the original boolean search terms.

Reference-to-PubMed transcriptors like "OssiPubMed online". O. Groth., which retrieves the PMID from one-letter coded journal abbreviations to get the full-text articles.

As most of these and other alternatives rely essentially on PubMed/MEDLINE data leased under license from the NLM/PubMed, the term "PubMed derivatives" has been suggested.[28] Without the need to store about 90 GB of original PubMed Datasets, anybody can write PubMed applications using the eutils-application program interface as described in "The E-utilities In-Depth: Parameters, Syntax and More", by Eric Sayers, PhD.[42]